The terahertz (THz) region is defined as electromagnetic (EM) radiation between frequencies of 100 GHz and 10 THz, corresponding to a wavelength range of 3 mm-0.03 mm. Interest in the THz region has been sparked by significant progress in the field of THz radiation detectors made in recent years. THz technologies have been applied in materials science, biology, biomedicine, medical spectroscopy, radar, communication systems and homeland security. However, while detectors are available, the most serious obstacle to the study and widespread use of THz applications is the lack of a reliable, tunable, solid state radiation source.
Frequency mixing is the nonlinear generation of the sum or difference between two radiation frequencies in a nonlinear medium. A special case is harmonic frequency generation, where the frequency of an intense single frequency radiation wave is doubled, tripled, quadrupled etc. In harmonic frequency generation, the nonlinear medium converts input radiation of a given wavelength λ0 to one of its harmonic wavelengths, such as λ0/2, λ0/3, λ0/4, etc. Harmonic frequency generation has been achieved in the optical region (e.g. by converting Nd-YAG laser radiation from λ=1.06 μm to λ=0.53 μm, λ=0.265 μm, etc.) using nonlinear crystals. Such a crystal is many optical wavelengths long (e.g. 1 cm), but if the generated radiation is phase matched (e.g. by quasi-phase-matching) to the pump laser wave, the harmonic wave grows constructively and coherently along the entire length of the crystal. We refer to this as frequency multiplication in an “EM distributed structure”. In the microwave region, e.g. at 10-100 GHz or λ=1-100 mm, harmonic frequency generation and mixing is usually achieved using discrete elements with dimensions smaller than a wavelength. This is done exemplarily using devices such as Schottky diodes, varactor diodes or, as recently proposed by Ong and Hartnagel (Semiconductor Science and Technology, Vol. 22 (2007) 981-987), quasi-ballistic electron reflection semiconductor devices. Such devices are “lumped” EM elements. Lumped EM elements do not require careful phase matching between the fundamental frequency input wave and the generated harmonic waves (as done in EM distributed structures). However, their power handling is limited to generation of harmonic powers below tens of milliwatts and their conversion efficiency is limited.
An integrated THz source has been suggested recently by Belkin, M. A. et al., Nature Photonics 1, 288-292 (2007). THz radiation is generated by mixing two separate mid-infrared frequencies within a semiconductor chip which includes two mid-infrared quantum cascade lasers (QCLs), forming a monolithically integrated structure. The source generates radiation at 60 μm (i.e. at about 5 THz), and can be tuned (by changing the output wavelength of one of the lasers). A major disadvantage of this source is that, at present, it only offers an estimated 100 nW of power when operated at 80K—far too small for most of the applications envisaged for the THz spectral range. This compares with the more than 10 mW of power commonly achieved in terahertz QCLs operating at cryogenic temperatures.
Another type of THz radiation source has been suggested by Hakimi in US Patent Application 2005/0242287A1. This source is based on radiative power conversion from a high frequency light source (laser) to THz radiation by means of a nonlinear frequency down-conversion process (Stimulated Raman Scattering (SRS) of Self Phase Modulation (SPM)). The down-conversion occurs in a polar material within a dielectric (core) optical waveguide. The dielectric guides the laser beam and the frequency-downshifted (Stokes) optical wave. The natural nonlinear material supports some solid state elementary excitation of discrete frequency (phonon, polariton, exciton, magnon or a quantum well level). The interaction of the two optical frequency waves (the pump and the Stokes wave) generates radiation at the beat frequency in the frequency range of the elementary excitation, which happens to be in the THz regime. This radiation is thus generated at the THz frequency range and is guided by a THz waveguide.